The present invention generally relates to an electromagnetic rotary drive and more particularly, to an electromagnetic rotary drive that functions as a bearingless motor-generator. Most particularly, the invention relates to a control system for a bearingless motor-generator.
Electromagnetic rotary drives are commonly used in standard motors as well as bearingless motor-generators. A conventional electromagnetic rotary drive includes a rotating part and a stationary part. The rotary part is commonly referred to as a rotor and the stationary part is commonly referred to as a stator. The stationary part supports a set of windings. A conventional control system for an electromagnetic rotary drive for a standard motor is shown in FIG. 1. The control system, generally indicated at 10, includes six switches 12, which control the flow of phase currents into a set of windings to produce forces, which exert a torque on a rotating part. However, this control system produces no forces for levitating the rotating part, since poles carry the same flux, and balance their forces out.
Unlike the standard motor described above, a conventional bearingless motor-generator produces forces for levitating the rotating part. An example of a bearingless motor-generator is described in U.S. Pat. No. 6,559,567, issued May 6, 2003, to Schob, the description of which is incorporated herein by reference. This bearingless motor-generator has a control system for an electromagnetic rotary drive that includes control devices, which control the flow of phase currents into two windings. The phase currents have a mutual phase shift of about 120°. The control system produces forces transverse to the windings. These transverse forces can be repulsive forces or attractive forces. By orienting the windings as described by Schob, the forces may be directed at an angle greater than 0° and less than 90° relative to the axis of rotation of the rotor. In this way, the rotor can be axially or radially levitated.
It should be noted that the bearingless motor-generator described above includes a drive winding for producing a drive field and a separate control winding for producing a control field. The drive field exerts a torque on the rotating part to rotate the rotating part and the control field exerts a force on the rotating part to levitate the rotating part.
A control system is needed that permits both drive and control fields to be produced from the same set of windings, thus eliminating the need to separate drive and control windings.